Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
A typical cellular wireless system includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs), such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station is typically coupled with equipment that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE operating within a coverage area of any base station can engage in air interface communication with the base station and can thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
In general, a cellular wireless system may be provided by a public land mobile network (PLMN) service provider and may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a “forward link” (or downlink) and communications from the UEs to the base stations defining a “reverse link” (or uplink). Examples of existing air interface protocols include CDMA (e.g., 1xRTT and 1xEV-DO), LTE, WiMAX, iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, MMDS, WI-FI, and BLUETOOTH. Each protocol may define its own procedures for initiation of calls, handoff between coverage areas, and functions related to air interface communication.
Further, air interface communications in each coverage area of a cellular wireless system may be encoded or carried in a manner that distinguishes the communications in that coverage area from communications in adjacent coverage areas. For example, in a CDMA system, each coverage area has a respective pseudo-random noise offset or “PN offset” that is used to encode or modulate air interface communications in the coverage area distinctly from those in adjacent coverage areas. And in an LTE system, each coverage area has a respective identifier (“cell ID” or “sector ID”) that is broadcast in a synchronization signal to distinguish the coverage area from adjacent coverage areas. Analogously, in other air interface protocols, communications in one sector may be distinguished from those in other sectors by frequency, time, and/or various other parameters.
To enable a UE to select an appropriate coverage area in which to operate, the base stations in a cellular wireless system may be arranged to broadcast in each of their coverage areas a respective pilot signal (or “reference signal”). Further, the cellular wireless system may have a particular network identifier, such as a PLMN ID or a carrier frequency on which the system operates, and the base stations in the system may broadcast their pilot signals using that identifier or may separately broadcast an indication of the network identifier (e.g., in an overhead message such as an LTE “master information block” for instance). In practice, a UE may then scan for and evaluate the strength of any detected the pilot signals, possibly preferring signals from a particular cellular wireless system for instance, and the UE may then seek to operate in the coverage area broadcasting the strongest pilot signal. For instance, the UE may transmit on a reverse link control channel in that coverage area a registration request or attach request, and the serving base station and associated equipment may process the registration and transmit on a forward link control channel a registration acknowledgement message. Further, once initially registered in a coverage area, a UE may continue to monitor the pilot signal of that coverage area as well as the pilot signals of adjacent coverage areas, to help ensure that the UE continues to operate in the coverage area providing the strongest coverage.
To initiate a call (e.g., voice call, data session, or other type of call) when operating in a coverage area, a UE may transmit on a reverse link access channel of the coverage area an origination request or the like, requesting the serving base station to allocate resources for the call. The base station may then reserve resources for the call and transmit on a forward link control channel a directive for the UE to proceed with the communication using particular air interface resources (such as a particular air interface traffic channel, timeslots, or resource blocks). Likewise, if the base station or associated equipment receives a request to connect a call to the UE, the base station may transmit on a forward link control channel a page message for the UE, and the UE may receive and respond to that page message to facilitate setup of the call.
During an ongoing call in a coverage area, the UE may also continue to monitor the pilot signal of that coverage area and the pilot signals of adjacent coverage areas. For instance, the UE may regularly measure signal strength from its serving coverage area and from adjacent coverage areas and may regularly report the signal strength level(s) in radio measurement reports to its serving base station. If appropriate, the base station or associated equipment may then arrange for handoff of the UE to another coverage area. For instance, if the UE reports a sufficiently stronger signal from an adjacent coverage area, the base station or associated equipment may arrange for transfer of the call to the adjacent coverage area and may send a handoff direction message to the UE, instructing the UE to continue the call in the adjacent coverage area.
Furthermore, in some cases, more than one air interface protocol might be implemented in a given market area. For example, a given market area might provide both 3G (e.g., CDMA 1xRTT and 1xEVDO) coverage under and also 4G (e.g., LTE or WiMAX) coverage. In a system that provides two or more air interface protocols in a single area, a UE might not only hand off between coverage areas under a common air interface protocol (e.g., between 3G CDMA coverage areas) but may also hand off between the different air interface protocols (e.g., between 3G CDMA coverage and 4G LTE coverage) within a given physical location or when moving between locations. Handoff between different air interface protocols is known as “vertical” handoff.
Vertical handoff may be appropriate in a scenario where a UE is operating under one air interface protocol and reaches a point where that air interface protocol does not provide adequate coverage. In that scenario, the UE may begin scanning for coverage of another air interface protocol (e.g., autonomously or at the direction of its serving network), and upon finding such other coverage may work with the serving network and/or the new network to orchestrate a handoff. Further, vertical handoff may be appropriate in a scenario where a UE is programmed to prefer one air interface protocol over another. In that case, when the UE is operating under the less preferred protocol, the UE may from periodically scan for coverage of the other protocol, and if the UE detects such coverage, the UE may then work with the serving network or the new network to orchestrate a handoff.